1. Field of the Invention
The present invention relates to an optical fiber amplifier and an optical fiber communication system. More specifically, the present invention relates to an optical fiber amplifier for amplifying a set of wavelength-division-multiplexed (WDM) signals with respectively different wavelengths with a flat gain, and an optical fiber communication system including a plurality of optical fiber amplifiers connected with each other.
2. Description of the Related Art
An optical fiber amplifier includes: an optical fiber doped with a rare earth element; a semiconductor laser pump source for optically pumping the rare earth element doped in the optical fiber; and an optical multiplexer for coupling the pump light (or a laser beam) emitted from the semiconductor laser pump source with the optical fiber doped with a rare earth element. When an input signal and the pump light are input to the optical fiber doped with a rare earth element such as erbium, a population inversion is generated by the pump light inside the optical fiber doped with a rare earth element, so that the optical signal is amplified by a stimulated emission.
Reflective optical fiber amplifiers utilizing the principle mentioned above are reported in Document 1 (Kobayashi, Ishihara and Goto, "Low Noise Configuration of Reflective Erbium Doped Fiber Amplifier", Proceedings of the 1993 IEICE Fall Conference, B-883, p. 4-124, 1993) and Document 2 (Asai and Takuma, "Modeling a Reflective Erbium-Doped Fiber Amplifier (EDFA); Illustration of Possibility of Improving Noise Characteristics", Optics New Era, Kogyo Tsushin, December, No. 49, pp. 23-25, 1993).
FIG. 7 shows a conventional optical fiber amplifier having such a configuration. As shown in FIG. 7, the optical fiber amplifier includes: an optical fiber 711 for inputting an optical signal; an optical fiber 714 connected with the optical fiber 711 via an optical circulator 713; and an optical fiber 712 for outputting an optical signal connected with the optical fiber 714 via the optical circulator 713. The optical fiber 714 is connected with an erbium-doped optical fiber 715 via a wavelength multiplexing coupler 717. Pump light emitted from a semiconductor laser pump source 718 is input though the wavelength multiplexing coupler 717 into one end of the erbium-doped optical fiber 715. A reflector 716 is provided at the other end of the erbium-doped optical fiber 715.
Next, referring to FIG. 7, the operation of the optical fiber amplifier will be described.
An optical signal propagated through the optical fiber 711 is turned by the optical circulator 713 to the direction as indicated by the arrow in FIG. 7, so as to be input to the optical fiber 714. The optical signal is then multiplexed by the wavelength multiplexing coupler 717 with the pump light emitted from the semiconductor laser pump source 718, so as to be directed to the erbium-doped optical fiber 715. While the optical signal is being propagated through the erbium-doped optical fiber 715, the optical signal is amplified. Then, the amplified optical signal is reflected by the reflector 716. The signal reflected by the reflector 716 is amplified again while being propagated through the erbium-doped optical fiber 715, so as to return to the optical fiber 714. Thereafter, the optical signal is turned again by the optical circulator 713 to the direction as indicated by the arrow in FIG. 7, so as to be output to the optical fiber 712.
However, if a set of optical signals with respectively different wavelengths are simultaneously input to such an optical fiber amplifier, then the gain spectra adversely become non-flat in accordance with the respective wavelengths of the signals. FIG. 8A is a graph showing the relationship between the input spectrum (the spectrum of each of the optical input signals) and each wavelength of the respective input signals, while FIG. 8B is a graph showing the relationship between the output spectrum (the spectrum of each of the optical output signals which have been amplified) and each wavelength of the respective output signals which have been amplified. In FIGS. 8A and 8B, optical signals with eight different wavelengths are input to the optical fiber amplifier.
As shown in FIGS. 8A and 8B, the plurality of signals are given non-flat gains at different levels in accordance with the respective wavelengths. Therefore, even though the input spectra are set to be equal, the output spectra of the amplified signals become different from each other in accordance with the respective wavelengths. This is because the absorption cross section and the emission cross section of the rare earth element (erbium in this example) doped into the optical fiber are varied depending upon the wavelengths, as shown in FIG. 9. Accordingly, this phenomenon is generated irrespective of the configuration of the amplifier to be used. In general, the gain G(.lambda.s) per unit length of an optical fiber doped with a rare earth element (erbium) with respect to the wavelength .lambda.s of the optical signal is expressed by the following equation using; an emission cross section .sigma.e(.lambda.s), an absorption cross section .sigma.a(.lambda.s), the number No of the ions pumped at a lower level, and the number Ne of the ions pumped at an upper level. The gains given to the optical signals do not become flat with respect to the wavelengths .lambda.s. EQU G(.lambda.s)=.sigma.e(.lambda.s).multidot.Ne-.sigma.a(.lambda.s).multidot.N o
This phenomenon seriously affects an optical fiber communication system for propagating a set of WDM signals by using an optical fiber and an optical fiber amplifier. For example, Document 3 (E. L. Goldstein, A. F. Elrefaie, N. Jackman, and S. Zaidi, "Multiwavelength Fiber-Amplifier Cascades in Unidirectional Interoffice Ring Networks", Technical Digest of Conference on Optical Fiber Communication (OFC/IOOC '93), TuJ3, pp. 42-44, 1993) reports non-flat gain spectra with respect to the wavelengths. In this document, fourteen WDM signals are propagated through fourteen optical fiber amplifiers cascaded by optical fibers.
In order to solve such problems, a method for compensating the gain difference with respect to the wavelengths by using the wavelength loss characteristics has been suggested. For example, Document 4 (M. Wilkinson, A. Beddington, S. A. Cassidy and P. McKee, "D-Fiber Filter for Erbium Gain Spectrum Flattening", Electronics Letters, vol. 28, No. 2, pp. 131-132, 1992) suggests a method for utilizing the wavelength loss characteristics of a diffraction grating provided in an optical fiber. On the other hand, Document 5 (S. F. Su, R. Olshansky, G. Joyce, D. A. Smith and J. E. Baran, "Use of Acoustooptic Tunable Filters as Equalizers in WDM. Lightwave Systems", Technical Digest of Conference on Optical Fiber Communication (OFC'92), ThC4, pp. 203-204, 1992) suggests a method for utilizing the wavelength loss caused by a supersonic wave in an optical modulator. However, according to either of the methods described in these documents, it is still impossible to completely equalize the gains.
As reported in Document 6 (Mitsuda, Ohya and Uno, "Analog Transmission Characteristics of a Bidirectional Optical Fiber Amplifier", Proceedings of the 1994 IEICE Spring Conference, C-398, 1994), when the difference in the levels of the optical signals with different wavelengths is large, a cross-talk phenomenon where the input of an optical signal with a long wavelength reduces the gain of an optical signal with a short wavelength. When such a phenomenon occurs, almost no gain is obtained in an optical signal with a short wavelength, as is disclosed in Document 5 based on the experiment results. In such a case, it is impossible to utilize the methods disclosed in Documents 4 and 5 for compensating for the gain difference with respect to the wavelengths by using the wavelength loss characteristics. In a conventional technique for propagating WDM signals, such a problem considerably affects the performance of an optical fiber amplifier.